The traditional monolithic RF base transceiver station (BTS) architecture is increasingly being replaced by a distributed BTS architecture in which the functions of the BTS are separated into two physically separate units—a baseband unit (BBU) and a remote radio head (RRH). The BBU performs baseband processing for the particular air interface that is being used to wirelessly communicate over the RF channel. The RRH performs radio frequency processing to convert baseband data output from the BBU to radio frequency signals for radiating from one or more antennas coupled to the RRH and to produce baseband data for the BBU from radio frequency signals that are received at the RRH via one or more antennas.
The RRH is typically installed near the BTS antennas, often at the top of a tower, and the BBU is typically installed in a more accessible location, often at the bottom of the tower. The BBU and the RRH are typically connected through one or more fiber optic links. The interface between the BBU and the RRH is defined by front-haul communication link standards such as the Common Public Radio Interface (CPRI) family of specifications, the Open Base Station Architecture Initiative (OBSAI) family of specifications, and the Open Radio Interface (ORI) family of specifications.
Wireless operators are under constant pressure to increase the speed, capacity and quality of their networks while continuing to hold the line on cost. As technologies evolve, the challenge is becoming increasingly difficult. One specific reason: the escalating occurrence and cost of passive intermodulation (PIM).
Already recognized as a significant drain on network performance and profitability, the problem of PIM is intensifying. Advanced wireless equipment is becoming more sensitive, and new technologies like LTE are increasingly noise limited. It has been noted that a 1 Decibel drop in uplink sensitivity due to PIM can reduce coverage by as much as 11 percent.
Testing for PIM using conventional coaxial RF testing equipment is slow, costly and dangerous. Each sector, frequency and technology must be individually connected and tested. So, most operators resort to PIM testing only after detecting a significant rise in the noise floor or a drop in connection quality. Therefore, improvements in PIM testing are needed so that operators can afford to make PIM testing a regular part of their network acceptance and preventative maintenance programs thereby increasing the profitability of their network in an increasingly competitive marketplace.
Measuring signal power from CPRI or any other digital interface which carries digitized RF/IF signal could be useful in quantifying the presence of interference power or the desired signal quality. To measure the signal power or quality, the important first step is to establish a reference. In radio receivers, the signal power is calculated from known reference of the full scale A/D voltage and the RF front-end gain. However, in the case of Optical PIM tester which interfaces to CPRI, the front end gain of RRH and the full scale A/D voltage are not known.